Years ago, when I was in graduate school, I got into an argument, about a real-world crime statistic, with another student who didn't have much math-sense. The mathematics itself is very simple, and yet at least one implication of it seems to run counter to the intuïtions of many people.

Let's say that a population p is divided into groups, each i-th group with population p_i p = ∑(p_i) And let's say that the i-th group has a propensity c_i to commit crimes, such that c_i · p_i gives the sum of the crimes committed (however measured) by members of that population.

If criminals from within each group draw their victims with each person having an equal chance of victimization regardless of his or her own group, then the proportionate share of victims that they draw from the j-th group will be p_j / p The total number of crimes then committed against the j-th group by members of the i-th group will then be (p_j / p) · (c_i · p_i) and the ratio of i-on-j crime to j-on-i crime will be [(p_j / p) · (c_i · p_i)] / [(p_i / p) · (c_j · p_j)] = c_i / c_j So, if c_i = c_j, then the ratio of i-on-j crime to j-on-i crime will simply be 1:1.

The other graduate student had been sure that, if group i were the smaller group, then the ratio should be larger than 1:1, because group j furnished more potential victims. The proper intuïtion here is that, if one group is larger than another, then it furnishes proportionally both more potential victims and more potential victimizers; or, to say the same thing differently, if one group is smaller than another, then it furnishes proportionally both fewer potential victims and fewer potential victimizers.

If we see a very different ratio, then the difference implies that one group has a greater propensity to criminality than the other, or that one group is seeking (or avoiding) the other in its acts of criminality, or both.

It should be noted that members of the j-th group may be sought or avoided for reasons other than their being members of that group as such. For example, members of the j-th group may happen to have more portable wealth. Still, if one sees a ratio of, say, about 50:1, then it's hard to explain this lop-sided ratio in terms simply of the j-th group having more wealth, or of the i-th group simply having a greater propensity to criminality. With a ratio like that, one should expect that members of the j-th group are indeed being targetted for being in that group, by members of the i-th group.

Controversy has erupted over Robert Downey jr having been cast in the rôle of a black character in Tropic Thunder. By coïncidence, over the last couple of days I have been watching The Most Dangerous Game (1932) in bits and pieces. The coïncidence is in that Noble Johnson, an African-American, played a Caucasian in that earlier film.

I don't think that we should give much of a d_mn about blacks playing whites or vice versa. It should be no more than a mild curiosity.

On the other hand, Tropic Thunder stars Ben Stiller, and the fact that he still has a career in movies certainly does offend me.

Addendum (09 Mar): I am now told, distinct from the report of the Daily Mail, that Downey plays a white actor playing a black character. (So the rôle would be somewhat more like that of the main character in Soul Man (1986).)